1. Field of Invention
This invention relates to marking devices, and in particular to ensuring precise registration between an original image and a reproduced image by correcting for deviations in the moving image receiving surface position/speed based on sensed data.
2. Description of Related Art
Ensuring that images in a marking system are accurately reproduced requires precise registration between the original image and the surface receiving the reproduced image. Particularly in the case of color images, in which some colors of the original image are matched by overlapping two or more primary ink colors, precise registration is important. Precise registration prevents the appearance of defects (e.g., a border between two colors not present in the original image) caused by slight misalignment of the marking element with respect to the corresponding area of the image receiving surface (e.g., a photoreceptor or a marking medium) at the time of the reproduced image is transferred.
Systems involving a moving image receiving surface (e.g., a belt or a drum) on which the original image is reproduced present even more difficult registration problems. In these systems, the transfer of the reproduced image must also be timed with respect to the advancing surface to ensure that the received image is in correct registration. Current standards require that images be registered within 15 .mu.m for DC registration shifts and as close as 2 .mu.m for AC registration shifts in the 1.0 to 1.5 cycles/mm range. Typically, DC registration shifts are errors that are constant throughout an image, whereas AC registration shifts are errors that change throughout the image.
As are known, various errors in a moving surface system can contribute to imprecise registration. In endless belt systems, there may be slight differences in belt speed at different points on the belt due to "runout" errors (i.e., errors caused by slight eccentricities in the rolls that support the belt). Similar errors may occur in rotating drum systems. As a result of runout errors, the velocity of a point on the image receiving surface may not be constant over an entire revolution of the image receiving surface.
In some moving surface systems, the moving surface includes registration marks that are optically sensed by the marking device to allow correction for small changes in the velocity of the moving surface by, e.g., varying the gearing slightly. See, e.g., commonly assigned U.S. Pat. No. 5,160,946. Using registration marks is undesirable in certain applications, however, because these marks must be removed so they are not visible in the final image.
According to one approach, as disclosed in European Patent Publication No. 062992 7A2 (EP 927) signals from a rotary encoder sensing device coupled to the rotating structure are used to measure displacement of the moving surface. The rotary encoder is mounted to rotate with one of the rolls that supports the belt or at another point in the belt circuit allowing accurate measurement of the moving surface. The rotary encoder records the displacement of the belt, and this information is transmitted to a circuit that controls and drives the rolls, allowing any necessary adjustments in roll speed (and thus, belt speed) to be made. Based on the feedback provided by the then rotary encoder, the motion of the moving surface is controlled by modulating an amount of delay such that images are transferred to the moving surface in proper registration.
As is disclosed in commonly assigned U.S. Pat. No. 5,119,128, which is incorporated herein by reference, the rotary encoder is positioned to rotate between a light source and an opposing photodetector. The rotary encoder is essentially a disk assembly with transparent areas and opaque areas. When the disk assembly is positioned such that one of the transparent areas is aligned between the light source and the photodetector, a light beam is transmitted through the transparent area and detected by the photodetector. When an opaque area of the disk assembly is aligned between the light source and the photodetector, the light beam is blocked, the photodetector senses the blockage of light, and the photodetector emits an electrical pulse. As the rotary encoder rotates, a series of such pulses are generated, and can be used to indicate the timing of the rotation of the roll and/or belt for coordination with other events. As a result, the number and position of the opaque areas on the disk assembly can be set to provide information on the relative angular displacement of the roll and/or belt.
The rotary encoder itself, as well as the particular roll or other rotating structure with which the rotary encoder is associated (hereinafter the "encoder roll"), have a runout error (hereinafter the "rotary encoder error") that causes potential registration problems. The component of the rotary encoder error due to the rotary encoder itself is generally greater than the component due to the encoder roll. Because the encoder roll is used to set the velocity of the moving surface and thus affects the entire system, correcting the rotary encoder error is highly desirable.
In EP 927, the rotary encoder error is corrected by correcting the period of each individual pulse output from the encoder. Multi-bit period time correction values corresponding to each individual pulse of the encoder signal are stored in a table. The period time correction values are the sum of a positive fixed time and a positive or negative corrective time. A corrected encoder signal is produced by a delay device that delays each pulse by its period time correction value.
If the approach to correcting the rotary encoder error requires storing values for all deviations from nominal values of a roll surface corresponding to each angular displacement (i.e., each line pair) of the rotary encoder, the table becomes large. As a result, memory requirements and processing speed increase.
Accordingly, it would be desirable to provide a rotary encoder error correction system and method to correct rotary encoder errors without requiring measurement and storage of all actual deviations for each rotational displacement.